Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

Abstract

We have performed ultrafast optical microscopy on single flakes of atomically thin CVD-grown molybdenum disulfide, using non-degenerate femtosecond pump-probe spectroscopy to excite and probe carriers above and below the indirect and direct band gaps. These measurements reveal the influence of layer thickness on carrier dynamics when probing near the band gap. Furthermore, fluencedependent measurements indicate that carrier relaxation is primarily influenced by surface-related defect and trap states after above-bandgap photoexcitation. The ability to probe femtosecond carrier dynamics in individual flakes can thus give much insight into light-matter interactions in these twodimensional nanosystems.There has been an explosion of recent interest in the quasi-two-dimensional (2D) transition metal dichalcogenides (TMDCs), which are layered materials with strong in-plane covalent bonding, leading to atomically thin layers within their structure 1 . Their unique 2D geometry allows for the design and fabrication of complicated structures at desirable positions within optoelectronic devices, much more efficiently than other low-dimensional nanomaterials 1,2 . In particular, molybdenum disulfide (MoS 2 ) has received much recent attention due to its excellent electronic 3 and optical properties 4,5 . Most notably, the indirect bandgap in the bulk (~1.2 eV) changes to a direct bandgap (~1.9 eV) as the number of atomic layers is reduced to one, causing MoS 2 monolayers to generate strong photoluminescence [4][5][6][7] . Therefore, unlike graphene, which does not have an intrinsic band gap 8 , TMDCs are especially promising for potential optoelectronic applications, such as photo-transistors 9 , photodetectors 10 , and electroluminescent devices 11 . Many of these applications will depend on a detailed knowledge of the optical properties and carrier relaxation dynamics, which can be elucidated by photoexciting carriers and probing their relaxation as a function of layer thickness with femtosecond time resolution.Here, we use ultrafast optical microscopy (UOM) 12,13 to measure ultrafast carrier dynamics in atomically thin single molybdenum disulfide flakes grown by chemical vapor deposition (CVD). By tuning the probe photon energy through the MoS 2 band gap (both indirect and direct), our UOM measurements show that conduction and valence band states are rapidly populated on a sub-picosecond (ps) time scale in a MoS 2 monolayer after photoexcitation at 3.1 eV, consistent with previous work [14][15][16][17][18] . Pump fluence-dependent measurements reveal that subsequent carrier relaxation in our samples is primarily due to surface-related defects and trap states, not the Auger processes observed in previous measurements on MoS 2 and other semiconductor nanosystems 12,[18][19][20][21][22][23] . We also observed an increase in the carrier relaxation time with an increase in the number of MoS 2 layers, likely due to the well known layer-dependent changes in the electronic structure 16,24 . Our UOM measurements of photon energy-a...